DEREK HOUGH returned to "Dancing with the Stars" last night after missing last week with a serious neck injury. He says his doctors have cleared him to finish out the season . . . but eventually, he's going to need neck surgery.

The "National Enquirer" claims a LOT of sex happens behind-the-scenes at "Dancing with the Stars" . . . and they're blowing the lid off of it. A so-called "production source" says, quote, "The number of affairs has gotten way out of hand. There's been so much casual sex behind the scenes that you'd think it's a swingers club, not a dance show." They posted a rundown of some of the alleged hook-ups, including:

Derek Hough: Shannon Elizabeth and Maria Menounos

Julianne Hough: Helio Castroneves and Chuck Wicks

Mark Ballas: Shawn Johnson and Sabrina Bryan

Maksim Chmerkovskiy: Karina Smirnoff, Willa Ford, Erin Andrews and Peta Murgatroyd

Cheryl Burke: Chad "Ochocinco" Johnson, Drew Lachey and Rob Kardashian